1. Field of the Invention
The present invention relates to liquid crystal-based video and graphics display devices, and, in particular, to sensing liquid crystal pixel current for such display devices.
2. Description of the Related Art
A substantial need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. In particular, wearable display devices could be used instead of the conventional displays of laptop and other portable computers. Potentially, wearable display devices can provide greater brightness, better resolution, larger apparent size, greater privacy, substantially less power consumption and longer battery life than conventional active matrix or double-scan liquid crystal-based displays. Other potential applications of wearable display devices are in personal video monitors, in video games and in virtual reality systems.
Miniaturized displays based on cathode-ray tubes or conventional liquid crystal displays have not been successful in meeting the demands of wearable displays for low weight and small size. Of greater promise is a micro display of the type described in U.S. Pat. No. 5,596,451 of Handschy et al. (digital pixel driver) and in European patent application no. 98122934.7 of Walker et al. (analog pixel driver), the disclosures of which are incorporated into this disclosure by reference. This type of micro display includes a reflective spatial light modulator that uses a liquid crystal (LC) material as its light control element. Typically, a ferroelectric liquid crystal (FLC) material is used as the light control element.
There is a need for a circuit that accurately senses liquid crystal pixel current. This is because the set voltage of the pixel electrode determines the reflectivity of the pixel. However, there is a time delay between the setting voltage and the optical reflection. If one does not compensate for the delay, the display will lose some contrast because the light source will turn on immediately even though the pixel does not. Sensing the liquid crystal pixel current provides a way to measure the time delay and thereby turn on the light (e.g., LED) to achieve best contrast (because the LC switches as the light is turned on).
One typical prior art approach to measure pixel current is to insert a resistor to detect current through current-to-voltage conversion. Using this approach one can sense the current through the liquid crystal with a resistive element connected between the pixel driver and the pixel electrode. The current is thus converted to a differential voltage. This is shown in FIG. 7. Unfortunately, the added resistive element changes the impedance between the pixel driver and the pixel electrode. This degrades the resulting image. Furthermore, linear resistors are not readily available in standard digital CMOS process. The use of nonlinear resistors can create significant nonlinear distortion when converting a current to a voltage.
Another typical prior art approach to measure pixel current is to use restricted areas for sensing the liquid crystal current. Using this technique, one attempts to sense the current through the liquid crystal in areas outside the pixel array. However, the current through the liquid crystal in these regions may not match the current through the liquid crystal material covering the pixel array.
Thus, it can be seen that modern liquid crystal pixel current sensing techniques impose contrast fidelity and production cost limits upon LC-based micro displays, and hinder the use of these micro displays in many applications.
Therefore, there is an unresolved need for an improved liquid crystal pixel current sensing technique that can increase LC-based micro display contrast fidelity and lower production costs.
A liquid crystal pixel current sensing technique is described that increases LC (Liquid Crystal)-based micro display contrast fidelity and lowers production costs.
A spatial light modulator has an electro-optical material (such as a liquid crystal layer) between an array of pixel electrodes and a common electrode (such as an indium tin oxide (ITO) layer). An amplifier biases the common electrode of the spatial light modulator and outputs a differential current proportional to the electro-optical material current. The output is used to determine delay of a light source to compensate for switching delay of the modulator.
For one embodiment, the amplifier has a common-source push-pull output stage and current mirrors. For another embodiment, the output stage is a cascode common-source push-pull output stage.
There are several advantages of this liquid crystal pixel current sensor. Because it does not perform current-to-voltage conversion, it does not alter the impedance between the pixel driver and the pixel electrode.
Also, it can be used to measure the current through the entire active display area.
Furthermore, this liquid crystal current-sensing circuit is a simple addition to the circuit that drives the ITO electrode with a DC bias voltage.
Moreover, this addition is very simple to design and requires very little extra area.